Boston will not be fully underwater in any mainstream climate scenario, but significant parts of the city will flood regularly and some low-lying areas face chronic inundation by the end of the century. The sea around Boston is projected to rise 9 inches by the 2030s, 21 inches by mid-century, and 36 inches (3 feet) by around 2070 to 2100, depending on how aggressively the world cuts emissions. That 3-foot rise has a greater than 50% chance of happening under moderate or business-as-usual emissions scenarios.
How Much the Sea Will Rise
Boston’s sea level rise projections come in three tiers, and the first is already baked in regardless of what happens with global emissions. Nine inches of rise is expected consistently across all scenarios and could arrive as early as the 2030s. Twenty-one inches is expected in the second half of the century across all scenarios as well. The highest planning figure, 36 inches, is the one that starts reshaping the city’s coastline. At 3 feet of rise, areas that currently flood only during major storms would flood at ordinary high tides.
These numbers only capture the ocean’s rise. Boston’s land is also sinking. NASA-funded research found that major East Coast cities, Boston included, sit on ground subsiding at 1 to 2 millimeters per year. That might sound trivial, but over decades it compounds. Part of the sinking traces back 12,000 years: when the massive ice sheet that once pressed down on northern North America melted, regions to the south began gradually settling downward in a geological rebalancing act that continues today. Human activity, including groundwater extraction and soil compaction, adds to the problem. Boston also shows “differential subsidence,” meaning some blocks are sinking faster than others, which strains roads, pipes, and building foundations unevenly.
What Flooding Will Look Like
The shift won’t arrive as a single dramatic event. It will show up as an increasing number of “sunny day” floods, where high tides push water into streets without any storm at all. NOAA projects that Boston will see 12 to 19 high-tide flood days in the 2025-2026 period alone. Nationally, coastal communities are expected to average 55 to 85 high-tide flood days per year by 2050. For Boston, that means flooded intersections, saltwater pooling in basements, and repeated disruptions to transit and traffic on a near-weekly basis within a few decades.
Storm surge is the bigger threat. When a nor’easter or hurricane coincides with high tide on top of elevated sea levels, flooding reaches much further inland. CoreLogic estimates that more than 24,000 multifamily properties in Boston are threatened by storm surge, worth roughly $9 billion in replacement costs. That makes Boston the third most exposed city in the country for multifamily housing.
Neighborhoods Most at Risk
Boston’s geography makes it uniquely vulnerable. Much of the city is built on filled land. Back Bay, the Seaport District, East Boston, and parts of Charlestown and South Boston sit on what was once harbor, marsh, or tidal flat. These areas are the lowest-lying and closest to the water, so they’ll see flooding first and most often. The Seaport District, one of the city’s most expensive and fastest-developing neighborhoods, is essentially a filled-in waterfront with elevations barely above current high-tide levels.
Flooding won’t stay at the coast. The Mystic River corridor, which runs through Somerville, Everett, Medford, and other communities north of downtown, is protected by the Amelia Earhart Dam. A 2018 feasibility study found that future sea level rise would overtop the dam, sending coastal flooding miles upriver. The state has committed $28 million to redesign and raise the dam by several feet, with construction expected to begin after 2026. Without that upgrade, neighborhoods as far inland as Arlington would face tidal flooding they’ve never experienced.
What Boston Is Doing About It
The city is pursuing protection on multiple fronts, from individual buildings to a potential harbor-wide barrier system. In 2021, Boston adopted the Coastal Flood Resilience Overlay District into its zoning code. Any new construction or major retrofit in areas expected to flood during a 1% chance storm in 2070 (with 40 inches of sea level rise) must now meet flood-resistant design standards. Projects go through a dedicated resilience review at multiple stages of permitting. This means new buildings in flood zones are being raised, waterproofed, and designed to handle conditions decades into the future.
For transit, the state secured $10 million in federal funds to install flood doors at the Blue Line tunnel portal near the airport. The tunnel connects to downtown, and a breach during a storm surge could flood miles of underground track and stations. The project is designed to protect the Blue Line against flooding through 2070.
The largest effort is a partnership between the City of Boston and the U.S. Army Corps of Engineers, now halfway through a six-year feasibility study expected to wrap up in 2028. The study is evaluating a full range of structural options: seawalls, floodwalls, surge barriers, elevated harborwalks, and nature-based solutions like restored wetlands. It’s also considering non-structural approaches such as zoning changes and evacuation planning. If a major structural project moves forward, it would be eligible for up to 65% federal funding. Draft project concepts for individual neighborhoods are currently being shared with the public for feedback, with a tentatively selected plan expected in winter 2026.
What This Means for Property and Daily Life
If you own property in Boston’s coastal neighborhoods, the trajectory is clear: flood insurance costs will rise, and properties in the most exposed areas will become harder to insure at all. The new zoning overlay means that if you’re buying a condo or home in the flood resilience district, the building should meet higher standards, but older buildings that predate the 2021 rules won’t have those protections unless they’re retrofitted.
For renters and residents, the practical impact is disruption. More frequent road closures, transit delays from water in tunnels, and occasional loss of power or sewer service during high-water events. Saltwater is corrosive to infrastructure in ways freshwater isn’t, so even after floodwaters recede, damage to underground pipes, electrical systems, and building foundations accumulates over time.
Boston won’t disappear beneath the waves. But the city that exists in 2060 or 2080 will relate to the ocean very differently than it does now. Some streets that are dry today will flood monthly. Some parks will become tidal marshes. Some buildings will sit behind new walls that didn’t exist a generation earlier. The question isn’t really whether Boston will be “underwater” but how much of its current layout survives intact, and how much money and engineering the city commits to keeping the water out.